1. Field of the Invention
The present invention relates to methods of rate detection at a receiving end of a digital communications system, such as a code division multiple access (CDMA) system, in which system the information data rate is variably selected at the transmitting end from an applicable rate set including a full rate and lower rates, each lower rate being the full rate divided by a different integer, and data is repeated for the lower rates to maintain a constant apparent data transmission rate. In its particular aspects, the present invention relates to a rate detection method in which a rate determination or classification decision is made using measurements of data derived from data frames decoded, after de-repetition for the lower rates, at a plurality of possible data rates in the applicable rate set.
2. Description of the Related Art
Such a rate detection method is generally known from xe2x80x9cAn Overview of the Application of Code Division (CDMA) to Digital Cellular Systems and Personal Cellular Networksxe2x80x9d, submitted by Qualcomm Incorporated to the Telecommunication Industry Association (TIA) TR45.5 Subcommittee on Mar. 28, 1992. At page 30 of this document it is stated that at the mobile station, the decoder must process the received frame four times, making decode attempts at each of the four data rates in the rate set, and the microcontroller must decide what rate was sent using Cyclic Redundancy Code (CRC) checking results and Symbol Error Rate (SER) measurements provided by the decoder. This document does not indicate how SER measurements could be provided by the decoder.
In 1992, a direct sequence code division multiple access (DS-CDMA) system was adopted as Interim Standard 95 (IS-95) by the TIA for deployment in the cellular band at 800 MHz. After successful field tests and trial systems the IS-95 system is now operating with, tens of millions of subscribers.
CDMA is based on spread spectrum technology originally developed by the Allies during World War II to resist enemy radio jamming. Spread spectrum signals are characterized by, a bandwidth W occupied by signals in a channel much greater than the information rate R of the signals in bit/s. Thus, a spread spectrum signal inherently contains a kind of redundancy which can be exploited for overcoming several kinds of interference (including signals from other users in the same band and self-interference in the sense of delayed multipath components) introduced by the channel. Another key property of spread spectrum signals is pseudo-randomness. Therefore, the signal appears to be similar to random noise, making it difficult to demodulate by receivers other than the intended ones. In CDMA systems, users share a common channel bandwidth and users are distinguished by different code sequences. In the case of IS-95 each communication with a user is modulated or scrambled by long and short Pseudo Noise (PN) sequences and also modulated by a specific one of a set of orthogonal sequences, known as Walsh codes, which is assigned to the user. The latter modulation is known as applying a Walsh cover. Thus, a particular receiver can recover a certain transmitted signal by applying the PN sequences, and also the Walsh sequence used by the corresponding transmitter for the particular receiver.
In the IS-95 DS-CDMA system variable information data rates are used according to the voice activity detected by the voice encoder. This enables a reduction in transmitted power at the lower rates leading to a reduced average transmitted power per user and consequent increase in capacity of the system. Two sets of information data rates (Rate Sets 1 and 2) can be encoded, depending on the implemented voice encoder each set comprising full rate, and lower rates of half rate, quarter rate, and eighth rate. For the lower rates, symbols are repeated to achieve the same apparent symbol transmission rate as when full rate is used. The information data rate can change from frame to frame, but information indicating the currently used data rate is not transmitted along with the speech data. Therefore, the receiver has to detect the data rate by hypothesis testing. The algorithm implemented by rate classification or decision logic which determines which of the possible information data rates is utilized for the current frame is called a Rate Detection Algorithm (RDA).
Available information which can be used for rate detection includes repetition characteristics, CRC checking results (which in accordance with IS-95 are available for all data rates except quarter and eighth rates in Rate Set 1), Viterbi decoder survivor metrics, and correlations between re-encoded data and data entering the decoder for each possible data rate. In the case of the latter, the symbols in each data frame are de-repeated (for the lower possible data rates), Viterbi decoded, and convolutional re-encoded in accordance with each of the possible data rates in the applicable rate set, and separate scaled correlations between the re-encoded data and the (de-repeated where required), data entering the Viterbi decoding are formed for each possible data rate.
An RDA employing correlations between re-encoded data and data entering the decoder for each possible data rate in an applicable rate set was employed in a CDMA cellular handset manufactured by Philips Consumer Communications, L.P. That algorithm had rate classification logic utilizing thirteen different thresholds or constants to distinguish between the data rates of IS-95 Rate Set 1 and also utilized Cyclic Redundancy Code (CRC) checking results for full and half rates.
It is an object of the present invention to provide an improved rate detection method in which correlations are formed between re-encoded data and data entering the Viterbi decoding for a plurality of possible data rates in an applicable rate set, which method employs rate classification logic having significantly fewer thresholds or constants per rate set than the previously used logic.
It is a further object of the invention that the rate detection method is highly accurate and reliable under expected operating conditions, such as the presence of noise and fading.
The foregoing and other objects are satisfied by providing such a rate detection method wherein the rate classification logic includes first determining whether or not the transmitted data rate is the full rate by checking if first conditions are satisfied, one of the first conditions being whether the full rate correlation plus a predetermined first threshold is greater than, or greater than or equal to, a maximum of the lower rate correlations. A further one of the first conditions is whether a Cyclic Redundancy Code check with respect to the decoded received convolution encoded data does not fail.
If the first determination is that the data rate is not the full rate, the rate decision logic proceeds to second determining whether or not the transmitted data rate is the largest of the lower possible rates by checking if second conditions are satisfied, one of the second conditions being whether the lower rate correlation for the largest of the lower possible rates plus a predetermined second threshold is greater than, or greater than or equal to, a maximum of the full rate correlation and the other of the lower rate correlation. A further one of the second conditions is whether a Cyclic Redundancy Code check with respect to the decoded de-repeated received convolution encoded data for the largest of the possible lower data rates does not fail.
Then, if the second determination is that the data rate is not the largest of the lower possible rates, the rate decision logic proceeds to third determining whether or not the transmitted data rate is the second largest of the lower possible rates by checking if third conditions are satisfied, one of the third conditions being whether the lower rate correlation for the second largest of the lower possible rates plus a predetermined third threshold is greater than, or greater than or equal to, a maximum of the full rate correlation and the other of the lower rate correlations. A further one of the third conditions is either whether the lower rate correlation for the second largest of the possible lower rates is greater than, or is greater than or equal to, a further predetermined threshold (for Rate Set 1) or whether a Cyclic Redundancy Code check with respect to the decoded de-repeated received convolution encoded data for the second largest of the possible lower data rates does not fail (for Rate Set 2).
Similarly, if the third determination is that the data rate is not the second largest of the lower possible rates, then the rate decision logic proceeds to fourth determining whether or not the transmitted data rate is the third largest of the lower possible rates by checking if fourth conditions are satisfied, one of the fourth conditions being whether the lower rate correlation for the third largest of the lower possible rates plus a predetermined fourth threshold is greater than, or greater than or equal to, a maximum of the full rate correlation and the other of the lower rate correlations. A further one of the fourth conditions is either whether the lower rate correlation for the third largest of the possible lower rates is greater than, or is greater than or equal to, a still further predetermined threshold (for Rate Set 1) or whether a Cyclic Redundancy Code check with respect to the decoded de-repeated received convolution encoded data for the third largest of the possible lower data rates does not fail (for Rate Set 2).
When there are three possible lower data rates, as in Rate Sets 1 and 2, and the result of the fourth determining is that the data rate is not the third largest (or smallest) of the lower data rates, the data is rejected as invalid.
The result is rate classification or decision logic with only six thresholds for Rate Set 1 and four thresholds for Rate Set 2. Such rate classification or decision logic is not only simpler than the previously used logic, but also simulations indicate that highly reliable rate decisions are made under expected operating conditions.